1. Field of the Invention
Solid detergents for dishwashing machines are sold predominantly in powder or granulate form. The individual particles of these detergents have a preferred diameter of from about 0.01 to about 3 mm. A serious disadvantage of solid detergents of this type, particularly those used institutionally with an alkali hydroxide content of from 10 to 60% by weight, is that, on account of the hygroscopicity of the individual raw materials, they show a pronounced tendency towards caking or clumping in the presence of small quantities of moisture.
By adding so-called "anti-caking" agents, such as paraffin for example, or by coating the highly hygroscopic constituents, such as for example the alkali metal hydroxides and the sodium metasilicate, with powder-form substances, such as for example pentasodium triphosphate or sodium sulfate, it is possible to obtain slight improvements in the caking or clumping behavior, although relatively large quantities of moisture in the form of water or water vapor still lead to caking or clumping of the powder-form or granulated detergents.
Although this does not make the detergents unuseable, because the effect of the individual constituents remains intact, even after clumping or caking in the presence of moisture, the appearance of the detergents in most cases is diminished or inferior in the eyes of the consumer, resulting in complaints.
The use of detergents which cake or clump in the presence of moisture is a particular disadvantage in automatic, dispenser-type metering units which are used in large numbers in institutional single-tank and multi-tank dishwashing machines. Metering problems arise either because too little detergent is dispensed or because the dispenser system itself is mechanically damaged and becomes unuseable. The only remedy is to prevent moisture from entering the storage compartment, although this cannot be guaranteed in the moist atmosphere of kitchens or after the machine has been cleaned with water without special drying of the storage compartment and the dispenser.
The clumping or caking of alkaline detergents can be avoided, inter alia, by preparing them in block form from the outset and packaging, marketing and using them in highly moisture-proof form.
2. Discussion of Related Art
U.S. Pat. No. 2,412,819 describes briquetted detergents for dishwashing machines which are produced by mixing together all the alkaline-reacting active substances preferably containing water of hydration, such as for example sodium silicates, pentasodium triphosphate, sodium hydroxide and, optionally, water if the preferred water of hydration in the compounds mentioned is not sufficient, and then gently heating the mixture with stirring to 90.degree. to 100.degree. C. until a uniformly molten mass is formed, pouring this melt into molds and allowing it to solidify into a compact crystal aggregate therein.
However, this does not in any way solve every problem because compounds containing active chlorine are extremely difficult to incorporate in strongly alkaline detergents.
Thus, European Patent 3 769 describes detergent containers, more especially for dishwashing machines, which contain solid detergent blocks produced from water and two solid constituents, at least one of which is an alkaline hydratable compound, and which are only open at one point from which the detergent is subsequently flushed out into the dishwashing machine by a stream of water. If, in addition to alkali metal hydroxides, these detergents are intended to contain active chlorine compounds which are regarded as necessary for the removal of bleachable soil, such as for example tea, coffee, fruit juices etc., they have to be introduced, preferably as preformed cores, into the soft mass during solidification of the detergent. In order to more effectively protect the chlorine donor, the core is said to be coated either with paraffin or with a wax-like mono- or dialkyl ester of polyphosphonic acid. The direct incorporation of compounds containing active chlorine, particularly in the presence of high concentrations of alkali metal hydroxide, is regarded as impracticable. In Example b 13, Table 1 of said patent it is shown that the direct addition of an active chlorine donor to the melt leads to a high loss of active chlorine in only 24 hours. The chlorine donors tested were Ca(OCl).sub.2, LiOCl and Na dichloroisocyanurate dihydrate. Residual chlorine activities of from only 3.5 to 17.4% of the original value were determined, depending on the chlorine donor.
Although the incorporation of preformed cores containing active chlorine donors is possible, such involves high costs through labor-intensive production and the necessary raw materials. In addition, the detergent blocks and their cores naturally differ in solubility.
Earlier German patent applications P 35 19 353.0 and P 35 19 355.7 describe a process wherein it is possible to work at low and, therefore, safe temperatures of from 45.degree. to 70.degree. C., and preferably from 45.degree. to 65.degree. C., providing the anhydrous alkali metal silicate as metasilicate hydrate and/or as waterglass solution, is first heated either by itself or together with solid alkali metal hydroxide or its monohydrate to 45.degree. to 48.degree. C. without influencing the resulting spontaneous heating to 60.degree. to 65.degree. C., all the other constituents, preferably containing water of hydration, including lastly, after the temperature of the melt has fallen to about 50.degree. to 55.degree. C., pentasodium triphosphate and/or its hexahydrate, organic complexing agents or an active chlorine donor are then introduced with stirring or kneading and the still liquid melt is poured into preferably flexible molds of any shape and allowed to solidify into blocks therein.
Therefore, an object of the present invention is to provide a process by which even active chlorine donors may optionally be incorporated in uniform distribution in block-form detergents in the presence of alkali metal hydroxides. At the same time, the invention seeks to provide detergent formulations which correspond to the present generation of standard powder-form or granular products for institutional dishwashing machines without the penta-alkali metal triphosphate jointly used being hydrolyzed and completely or partly degraded to the orthophosphate.